Electronic systems include a number of components, each of which performs various functions. These components must be interconnected either by connecting individual components together and/or by connecting the individual components to a data bus. Various different types of buses have been developed depending on the type of components to be connected and their data rates. For example, a parallel bus includes multiple data lines allowing simultaneous transfer of multiple bits of data. In contrast, a serial data bus includes only a single data line and may optionally include a clock line to ensure proper decoding of data.
The IEEE-1394 standard provides for a high-speed serial bus and has been broadly adopted by the consumer electronics (CE) industry as the single bus standard to unify communications between emerging all-digital CE devices. Due to the IEEE-1394 standard's small connectors and serial data cables, IEEE-1394-enabled devices are consumer friendly. Furthermore, the IEEE-1394 standard's protocol supports plug-and-play operation. As the IEEE-1394 standard becomes available on more CE devices, such as a digital video recorder 102, a digital cable set top box 104, a digital television 106 and an audio source 108, the devices being daisy chained together as shown in FIG. 1 as part of an IEEE-1394-based network 100. These audio and video devices may be connected to a personal computer 110, though this is not required. The personal computer itself may include peripheral devices such as an external hard disk drive 112 and a printer 114, to which a digital camera 116 may be attached.
As part of the IEEE-1394 standard, which is hereby incorporated by reference, individual devices can be placed into a suspended state to conserve energy. For example, when data is no longer required from the digital video recorder 102, the personal computer 110 can place the digital video recorder 102 in a suspended state. When data is later required from the digital video recorder 102, the personal computer 110 will send a resume command to the digital video recorder 102 and data can once again be received from this source by the personal computer 110.
For devices operating under the IEEE-1394-95 or -1394a standards, or for a device operating under the IEEE-1394b standard with a port in data strobe (D/S) mode, the loss of input bias causes the device's PHY port to transition into a suspended state. A problem arises when the loss of input bias is only momentary and not intended to transition the device into a suspended state.
A momentary loss of input bias that transitions the device into a suspended state has traditionally required one of two actions to resume data communication between devices. The first action is to physically unplug the IEEE-1394 serial data cables between the devices. This manual intervention is undesirable and can lead to consumer dissatisfaction. Such manual intervention is especially troublesome when a number of devices are daisy chained together. As a consumer will not know which link in the daisy chain caused the problem, the consumer may be forced to unplug and reconnect each of the IEEE-1394 serial data cables.
The second action that can cause a device in a suspended state to resume data communication requires the use of a smart controller. In such a system, a smart controller, a personal computer 110 for example, polls each of the devices and determines if any are unintentionally in a suspended state. If any devices are found to be in an unintentional suspended state, a resume command is sent. The disadvantage of this approach is that it requires the system to have a smart controller, which will not always be the case. For example, connecting a digital video recorder 102 to a digital television 106 for playback of recorded videotapes does not require a personal computer 110. In this case, no smart controller is present and neither device can transmit the required resume command. The consumer is therefore forced to physically unplug and reconnect the IEEE-1394 serial data cable.
When a device operating in the D/S mode encounters noise spikes on the input bias, it may interpret these spikes as a command to transition into a suspended mode. The nominal input bias voltage could be as low as 1.165 V while the threshold voltage for detecting loss of input bias can be as high as 1.0 V. A voltage drop of more than 0.2 V can occur when new devices are connected to the daisy chained network. This is especially likely to occur due to the in-rush of current required to drive the new device when the IEEE-1394 serial data cable powers the new device. One solution to this problem is to low-pass filter the line to remove these bias drops or noise spikes. However, as noted in the IEEE-1394a standard (see section 4.4.4), this filter must have a relatively short time constant of 200-300 ns. Unfortunately, bias drops and noise spikes have been observed empirically to have durations much longer than this.